The present invention relates to camera lighting systems, and more particularly relates to an apparatus and method for illuminating articles moving beneath a camera on a conveyor belt.
Automatic conveyor systems have been developed for handling and sorting articles, such as parcels or components of manufactured goods. The articles bear identifying indicia, for example, bar codes, dense two-dimensional symbols, and text suitable for optical character recognition. An overhead linear CCD (Charge Coupled Device) camera can capture narrow images of a field of view across a xe2x80x9cscan linexe2x80x9d that is very short in the direction of conveyor travel, but extends across the conveyor. A computer receiving the output of the camera can build from these linear images a full digital image of a parcel or a label passing under the camera. When the articles have varying heights, the camera can take an image anywhere in a xe2x80x9cscan planexe2x80x9d projected downward from the camera to the scan line.
Prior illumination systems have produced an intensive strip of light along the scan line by using elliptical cylinders as reflectors behind an elongated tubular lamp. The lamp lies along one focal axis of the ellipse, while the surface to be illuminated lies near the other focal axis. The light source may be inclined about the axis of the scan line so as not to obstruct the camera""s field of view. Generally, the region of focused bright illumination is not vertically deep within the scan plane, and thus the efficacy of this configuration is limited when the articles passing beneath the camera vary widely in height.
Such an illumination system seeks to direct enough light reflected from the subject article to the camera to obtain a sharp image, without blinding the camera with glare. Thus, the light sources should provide a large amount of diffuse reflection to the camera, but no specular reflection. However, articles with shiny upper surfaces present a particular challenge. If the angle of inclination of the light source about the scan line is small, the camera may be blinded by glare reflected from glossy packages or from plastic protectors used to protect paper labels. If the angle of inclination is large, tall articles may cast shadows onto leading or trailing articles when they are under the camera. At an optimum angle close to forty-five degrees, the light source can cover only a relatively small depth vertically in the scan plane, because elliptical reflectors cast a narrow beam transverse to the length of the reflector. One prior system provides an unsymmetrical elliptical reflector to illuminate a vertical region in the scan plane, but this approach does not solve all the problems noted above.
Patents disclosing illumination systems include U.S. Pat. Nos.:
There is a need in the art for an illumination system capable of providing adequate diffuse light to an overhead camera, reflected from a vertical region sufficient to accommodate a range of article heights. The illumination system should avoid reflecting glare to the camera, and should direct light so that tall articles do not cast shadows into the illuminated region.
The present invention seeks to provide a system and method for illuminating a surface of an article being imaged by a camera so as to reflect diffuse light to the camera from articles of varying heights.
In accordance with the invention, this object is accomplished in an illumination system by providing an apparatus for illuminating a surface of an article being imaged by a camera, comprising an elongated light source angled with respect to the surface of the article and positioned alongside the article to direct light toward the surface; and a light directing device positioned to receive light from the light source and to direct the light toward the surface at an angle equal to or greater than forty degrees from an optical axis of the camera. By constraining essentially all of the light rays to angles 40 degrees or more from the optical axis of the camera, the system avoids a level of specular reflection or glare that would deteriorate the image of symbols or text on the surface of the articles to the extent of preventing reliable decoding or reading of such symbols or text. When the invention is incorporated in a conveyor system in which the articles are moved on a conveyor, the light source is positioned alongside the conveyor to direct light angled downwardly and from the side of the conveyor. The light directed by the light directing device forms an illuminated strip on an upper surface of the articles transverse to their direction of travel.
In one embodiment of the invention, the light source is an elongated lamp and an elongated reflector positioned to reflect light from the lamp toward the articles, and the light directing device is a Fresnel lens. Preferably, the Fresnel lens has an optical axis that is offset from an axis of symmetry of the light source. To avoid specular reflection into the camera, the Fresnel lens bends light from the light source to a larger angle with respect to the optical axis of the camera. The optical axis of the lens is offset from the axis of symmetry of the light source away from the article along the direction of the optical axis of the camera to more efficiently illuminate both short and tall articles. A barrier, such as a housing surrounding the light directing device, may be used to block light from the light source except light passing through the light directing device.
The invention also provides an automatic conveyor system in which the longitudinal axis of the lamp is approximately perpendicular to the path of travel of articles along the conveyor. The reflector preferably is an elliptical surface and the lamp lies along a first focal axis of the elongated reflector. The second focal axis of the reflector preferably lies a distance beneath the conveyor. In the preferred embodiment, the Fresnel lens is positioned a distance from the lamp from about xc2xc to about xc2xd times the focal length of the Fresnel lens and a distance from a central axis of the conveyor approximately equal to 1 to 2 times the focal length of the Fresnel lens, and the optical axis of the Fresnel lens is offset from the axis of symmetry of the reflector by a distance from about 0.5 to 1 times the length of the lamp filament.
In a preferred embodiment, the illumination system includes a plurality of illumination assemblies, including light sources and light directing devices, positioned to provide light to the illuminated region of the camera""s scan plane. Optimally, four such assemblies are provided, the first and second assemblies on one side of the conveyor, one above the other, and the third and fourth assemblies on the opposite side of the conveyor, one above the other. When only one illumination assembly is present, preferably its symmetry plane substantially coincides with the camera""s scan plane. When a plurality of illumination assemblies are used, they may be aligned with the camera""s scan plane to maximize illumination. Optionally, they may be positioned with their symmetry planes slightly mis-aligned to create a thickness in the illuminated region spanning the scan plane, and thus in the illuminated strip on the surface of the article.
In preferred embodiments of the invention, the lamps, reflectors, lenses, and barriers are selected, assembled and oriented such that the articles are illuminated from the side of the conveyor, such that essentially all of the light exiting the lens is aligned at an angle equal to or greater than forty-five degrees from the optical axis of the camera, and such that diffuse reflection but essentially no specular reflection from the articles reaches the camera. Because the light sources are to the side of the conveyor, tall articles cannot cast shadows on leading or trailing articles. The preferred arrangement of multiple light sources illuminates a deep region of the scan plane without the need to position any light source so high as to create glare into the camera.
The present invention also provides a method for illuminating articles moving on a conveyor in a direction of travel under a camera, comprising the steps of directing a beam of light from the side of the conveyor at a downward angle toward the articles; and collimating the beam to direct the light at an angle equal to or greater than forty degrees from an optical axis of the camera and to form an illuminated strip on an upper surface of the articles transverse to the direction of travel.
In an alternative embodiment, a pair of additional illumination assemblies may be installed within the footprint of the camera""s field of view, if necessary for tall articles. In another alternative configuration for any of the above embodiments, the reflector of the light source may be part of a cone-shaped figure, rather than part of a cylinder. As a result, the lamp is inclined with respect to the second focal axis of the reflector. This configuration allows the light to be focused parallel to the scan line despite the inclination of the lamp.
In another alternative embodiment, the light directing device can be a set of multiple apertures forming narrow slots across the reflector, under the lamp and perpendicular to the filament axis. Reflective and diffusing surfaces within the slots are configured to avert direct illumination of the articles by beams having an angle of incidence less than forty degrees from the camera""s optical axis, and to create useful diffuse light from undesirable direct light beams.